A forage harvester usually is equipped with a detachable crop collecting apparatus, such as a row crop attachment or a pick-up device, which takes the crop off the field and feeds it to the inlet of the front unit of the forage harvester. In order to obtain good chopping quality, i.e. comminution of the crop material to a constant chop length, an even mat of crop material has to be fed at a constant speed towards the cutterhead to be comminuted between the rotating knives of the cutterhead and a stationary shearbar. The chopped material is fed into a blower unit with paddles, which throw it up into a discharge spout for collection in a cart or container.
Commonly, the mat of crop material is formed between stationary lower feedrolls and movable upper feedrolls, which are mounted for vertical displacement inside the front unit. The upper feedrolls are forced downwardly by a pair of springs for compression of the crop incoming material. Retainer means have to be provided to prevent interference of the upper and lower feedrolls when no material is being fed into the forage harvester and the upper rolls are in their lowermost position. Furthermore the shafts of the feedrolls which extend through apertures in the feedroll housing, must not contact the edges of the apertures to prevent undue wear of the components. Hence a supporting rim below these shafts cannot be used for limiting the downward travel of the feedrolls.
Most commonly abutments are provided on the feeder frame below the bearing housings or gearboxes of the upper feedrolls for halting the latter in a position suitably remote from the lower feedrolls. Such abutments may be equipped with rubber bumpers to cushion the downward motion of the feedrolls. Alternatively bumpers may also be provided on the bearing housings or gearboxes.
It has been experienced that the flat surfaces of the abutments have to be cleaned at regular intervals in order to preserve the original minimum distance between the upper and lower feedrolls. Otherwise the accumulation of stray crop material and dirt on the abutments prevents that the upper feedrolls return to their original lowermost position. Under these circumstances the feedrolls are no longer capable of providing a regular flow of crop material to the cutterhead as soon as the flow rate decreases and the thickness of the mat between the feedrolls diminishes accordingly. Consequently the cutterhead is loaded in an irregular manner and no constant chop length can be realized or maintained.